Preheat oven to moderate, 180°C.
Heat the oil in a medium frying pan on high. Cook mushrooms for 4-5 minutes, until golden. Drain mushrooms on paper towel, reserving oil in pan. Cool.
In a bowl, whisk together eggs, ½ cup of the cheese, milk, olives, salami and mushrooms. Season to taste.
Heat reserved oil in pan on low. Add egg mixture. Cook for 4-5 minutes, until frittata sets around the edges.
Place pan in oven. Bake for 10-15 minutes, until just set. Sprinkle with remaining cheese. Cut into wedges. Serve with mixed leaves.